1. Field of the Invention
The present invention concerns display panels using luminous elements such as light-emitting diodes, for example.
2. Description of the Prior Art
A serious disadvantage of known display panels is their poor visibility under high illumination, for example during very sunny weather in the case of outdoor panels.
In this situation it is common for the illumination due to the sun to be as high at 10 000 to 50 000 lux, that is to say 1 to 5 lumens per square centimeter.
One should consider the example of a display panel having a mosaic structure made up of matrices or bars of light-emitting diodes each adapted to deliver a few tens of millilumens through windows having a surface area in the order of one square centimeter, surrounded by a background in front of which is disposed, slightly forward of the light-emitting diodes, a perforated mask through the holes in which the light emitted by the light-emitting diodes passes, and which is required to be totally black. In this example, it is found that the illumination of the luminous areas corresponding to the holes in the mask and due to the luminescence of the diodes (and therefore amounting to a few tens of millilumens per square centimeter) is not preponderant as compared with the ambient illumination (30 to 200 millilumens on the black areas, since it is difficult to reduce reflections here to less than 3 or 4%).
The conventional way to minimize this disadvantage is to place in front of the perforated mask (on the observer side) a filter which generally comprises a plate of synthetic material the same color as is emitted by the luminous elements; most of the light emitted by the luminous elements is therefore transmitted to the observer whereas only the component of the ambient light which corresponds to the color of the filter is transmitted to the display surface, which reduces the illumination due to ambient light. The light-emitting diodes that are tending increasingly to be used emit in the yellow-orange band. The human eye is highly sensitive to the yellow-orange band, so that the display is as legible as possible in the usual cases. The yellow-orange band is particularly strong in solar radiation, however, which means that in this band the filter (which is also yellow-orange) is relatively ineffective and cannot in practice significantly enhance the legibility of the panel.
Another conventional way to attempt to increase legibility is to increase the power of the light-emitting diodes, or the other luminous elements used.
However, this approach comes up against technological obstacles that are increasingly difficult to overcome.
An object of the invention is to remedy this disadvantage.